The present invention relates to a semiconductor device, and more particularly to a semiconductor device with an insulation film having emitter contact windows filled with polysilicon films.
It has been known in the art to which the invention pertains that, in order to form a shallow emitter-base junction of a bipolar transistor, a polysilicon film having contained an impurity is formed on a base region for subsequent impurity diffusion into selected parts of the base region so that emitter regions having the diffused impurity are selectively formed in the base region. The above impurity doped polysilicon film is formed over an inter-layer insulator and also within emitter contact windows having been formed in the inter-layer insulator and positioned over an emitter-formation region in the base region. Since the emitter-base contact is formed in fine pattern, it is required to provide a check region for allowing required check of device characteristics and performances during the manufacturing process.
One of conventional emitter contact window structures for allowing required check of device characteristics and performances during the manufacturing process is disclosed in Japanese laid-open patent publication No. 58-58760. Descriptions of the conventional emitter contact window structure of the semiconductor device will be made with reference to FIGS. 1A and 1B. FIG. 1A is a fragmentary plane view illustrative of a semiconductor device having the conventional emitter contact window structure. FIG. 1B is a fragmentary cross sectional elevation view illustrative of a semiconductor device having the conventional emitter contact window structure taken along an "c--c" line of FIG. 1A. Probes are made into contact with a polysilicon layer 5 and a base contact window for checking various characteristics of the semiconductor device such as high voltage performances and current amplification coefficient thereof during a diffusion process before metal electrodes will be provided. Each of emitter contact windows 4 is formed in slit-shape which has the same width as a width of an emitter region which is intended to practically serve as emitter of the bipolar transistor. The emitter contact windows 4 are to allow checking the various device characteristics. Emitters are formed under the emitter contact windows 4 for allowing the required checking of the device characteristics. Since the emitter contact windows 4 have the same width as the width of the actually serving emitter region, then the emitter-base junction formed on the check region has the same in curvature and in emitter injection efficiency as the emitter-base junction formed in the device region. This results in a reduction in relative variation in properties between the check pattern and the device portion. This allows the characteristics of the device to be accurately controllable.
The above first conventional emitter contact window structure is, however, engaged with a problem with limitations for scaling down to the semiconductor device and to increase the integration of the semiconductor device.
It has also been proposed that the impurity doped polysilicon film is formed to be filled within emitter contact windows having been formed in an inter-layer insulator but not extends over the inter-layer insulator. This conventional technique is disclosed in Japanese laid-open patent publication No. 3-253033. This second conventional structure allows a narrow distance between the emitter-base junction and the base-collector junction. This second conventional structure is, therefore, capable of settlement of the above problem with the limitations to scaling down of the semiconductor device and to increase the integration of the semiconductor device. Fabrication processes for a semiconductor device with the second conventional structure will be described. FIGS. 1A through 2F are fragmentary cross sectional elevation views illustrative of semiconductor devices with the second conventional emitter contact window structure in sequential steps involved in a conventional fabrication method.
With reference to FIG. 2A, a silicon substrate has a check region "A" and a device region "B". Base regions 2 are formed in selected upper regions of the silicon substrate 1. An insulation film 3 is formed which entirely extends over the silicon substrate 1 and the base regions 2. A reactive ion etching is carried out by use of a resin film as a mask for selectively forming emitter-contact windows 4 and base contact windows 8 in the insulation film 3. Subsequently, a polysilicon film 5 is deposited entirely over the surface of the substrate so that the polysilicon film 5 extends over the base regions 2 and the insulation film 3. During the growth of the polysilicon film 5, an impurity is introduced into the polysilicon film 5. A resin film 9 is further formed over the polysilicon film 5. A spin-on-glass film 10 is entirely formed over the resin film 9. A resin film 11 is selectively formed on the spin-on-glass film 10 on an emitter-formation region in the check region "A".
With reference to FIG. 2B, a reactive ion etching is carried out by use of the resin film 11 as a mask to selectively etch the spin-on-glass film 10.
With reference to FIG. 2C, a reactive ion etching is carried out by use of the spin-on-glass film 10 as a mask to selectively etch the resin film 9 and the polysilicon film 5 so as to leave the resin film 9 over the emitter-formation region in the check region "A" and also leave the polysilicon film 5 within the emitter-contact windows 4 and the base contact windows 8 and also under the spin-on-glass film 10.
With reference to FIG. 2D, the spin-on-glass film 10 is removed by an etchant of diluted fluorine acid solution and subsequently the resin film 9 is removed in an oxygen plasma. A resin film 12 is selectively formed which covers the polysilicon films 5 within the emitter contact window 4. The polysilicon films 5 within the base contact window 8 are removed by use of the resin film 12 as a mask wherein a nitric system solution is used as an etchant.
With reference to FIG. 2E, the resin film 12 is removed in the oxygen plasma before a heat treatment is carried out to cause an impurity diffusion from the polysilicon film 5 remaining within the emitter-contact window 4 into the base region 2 whereby emitter regions 7 are selectively formed within the base region 2.
The above process shown in FIG. 2E can be replaced by a process shown in FIG. 2F.
The above second conventional problem is, however, engaged with a disadvantage in the requirement for additional processes for forming the check regions. Namely, the resin film 11 is selectively formed for allowing the polysilicon film 5 to remain within the emitter contact window 4.
In the above circumstances, it had been required to develop a novel emitter contact window structure for a semiconductor device free from the above problems and disadvantages.